Liquid cooled dummy load for RF transmission line

ABSTRACT

A liquid cooled dummy load device or termination for an RF coaxial transmission line. Electrical energy is converted to heat energy in a resistive film that is series-connected between the inner and outer conductors of the transmission line and deposited on a tubular cylindrical dielectric substrate. Liquid coolant being circulated through the device to and from a heat exchanger is moved in an axial direction through an inner annular flow passage defined by the inner cylindrical surface of a low friction tube and by the resistive film to absorb the heat energy, then outwardly through radial ports to an outer coaxial annular flow passage defined by the outer cylindrical surface of the aforesaid tube and by the inner surface of another low friction tube and finally to an outlet fitting. The housing has an interior surface form defining an exponential horn chamber surrounding and coaxial with the resistive film to minimize reflection from the device.

[451 Sept. 16, 1975 LIQUID COOLED DUMMY LOAD FOR RF TRANSMISSION LINELeo Lesyk, Walton Hills, Ohio [73] Assignee: Bird ElectronicCorporation, Solon,

Ohio

[22] Filed: Dec. 13, 1974 [2]] Appl. No.: 532,724

Related US. Application Data [63] Continuation-impart of Ser. No417,104, Nov 19,

1973, abandoned.

[75] Inventor:

[30] Foreign Application Priority Data Oct. 25. 1974 Canada 212346 Nov.14, 1974 Germany 2453962 Nov. 19, 1974 United Kingdom 50103/74 52 US.Cl. 333/22 F; 333/81 A 51 m. cm H01? l/26 [58] Field of Search 333/22 R,22 F, 81 B, 81 A l/l972 Lesyk et al. 333/22 F Primary ExaminerPaul L.Gensler Attorney, Agent, or Firm-Bosworth, Sessions & McCoy 57 ABSTRACTA liquid cooled dummy load device or termination for an RF coaxialtransmission line. Electrical energy is converted to heat energy in aresistive film that is series-connected between the inner and outerconductors of the transmission line and deposited on a tubularcylindrical dielectric substrate. Liquid coolant being circulatedthrough the device to and from a heat exchanger is moved in an axialdirection through an inner annular flow passage defined by the innercylindrical surface of a low friction tube and by the resistive film toabsorb the heat energy, then outwardly through radial ports to an outercoaxial annular flow passage defined by the outer cylindrical surface ofthe aforesaid tube and by the inner surface of another low friction tubeand finally to an outlet fitting. The housing has an interior surfaceform defining an exponential horn chamber surrounding and coaxial withthe resistive film to minimize reflection from the device.

3 Claims, 4 Drawing Figures PATENIEB SEP I6 3975 LIQUID COOLED DUMMYLOAD FOR RF TRANSMISSION LINE CROSS REFERENCE TO RELATED APPLICATIONThis application is a continuationimpart of prior application Ser. No.417,104 filed Nov. 19, 1973, now

abandoned.

BACKGROUND OF THE INVENTION This invention relates to dummy load devicesfor use as reflectionless terminations for RF coaxial transmissionlines. Morepartieularly the invention relates to liquid-cooled dummyload devices which rely on convection to dissipate heat energy. In thistype of device a liquid cooling, medium is circulated through the deviceto carry off the heat energy generated by the dissipation of electricalenergy across the load. Normally a heat exchanger and pump are providedto minimize the volume of cooling water required.

Frequently in testing transmitter apparatus or in measuring radiofrequency power, a substantially reflectionless termination or dummyload is used to terminate the coaxial transmission line. The terminationmust be capable of absorbing and dissipating the power in the form ofheat. Also the termination must be matched to the electricalcharacteristics of the coaxial transmission line asdetermined by thephysical dimensions of the line in order to avoid reflection of radiofrequency waves from the termination. In order to minimize reflectionand maximize power transfer from a transmission line to a termination,the load should have a characteristic impedance that is matched to thecharacteristic impedance of the line. For coaxial lines thecharacteristic impedance is defined by: Z,,= I L/C where zfljharacteristic impedance L=Distributive inductance C=Distributivecapacitance More conventional types of terminations employed for coaxiallines utilize a tapered horn principle to minimize reflection. Themicrowave signals are passed along a resistive layer defining thetapered surface, and the signal is thus gradually attenuated in anadvantageous manner to minimize reflection. Typical examples of suchline terminations are disclosed in US. Pat. Nos. 2,556,642; 2,752,572;2,984,219; 3,300,737; 3,213,392 and $634,784.

In a typical construction for the devices disclosed in the above patentsa tubular cylindrical ceramic mem' ber with a resistive filmor coatingapplied to the exterior cylindrical surface is mounted in the device toprovide the electrical load. The device normally has one end adapted formaking the electrical connection to the transmission line and the otherend adapted for connection to the inlet and outlet lines for the liquidcoolant. As indicated above, the member with the resistive film coatedthereon is hollow and a coaxial logarithmic horn is provided around theresistor to minimize reflection. i

Liquid coolant normally is supplied to the interior of the ceramicmember at the end with the inlet fitting and flows therethrough to theopposite end and then outward such as through radial ports in theceramicsubstrate toan annular flow passage that surrounds the resistivefilmlThe liquid coolant then reverses flow and proceeds back to theopposite end of the device to the outlet fitting as it absorbs heatenergy generated by the resistive film. Thus the circulation of liquidcoolant proceeds first through the interior of the ceramic element andthen outward and across the surface of the resistive film.

Normally the liquidcoolant must be pumped at a relatively high velocitybut in view of the relatively large amount of water within the tubularceramic element there is considerable resistance to fluid flow when thewater moves outward and along the annular flow path surrounding theresistive film. In other words, the path ofcirculating in the aboverecited type of construction generates a substantial pressure drop. Alsocontributing to the pressure drop is the surface friction resistancebetween the cooling liquid and the ceramic sur face defining therespective flow passages. Ceramic surfaces generally have a fairly highsurface friction characteristic which contribute significantly to theoverall flow resistance.

The device of the present invention, however, reduces the disadvantagesdescribed above and affords other features and advantages heretofore notobtainable.

SUMMARY OF THE INVENTION It is among the objects of the invention toimprove the heat transfer in a liquid cooled dummy load device.

Another object is to minimize resistance to fluid flow (i.e., reduce thepressure drop) in a dummy load device for an RF coaxial transmissionline.

These and other objects and advantages are achieved by the liquid cooleddummy load device of the present invention which includes a cylindricalhousing formed of conductive material and a connector at one end of thehousing adapted to receive a mating connector from the coaxialtransmission line. An elongated tubular dielectric member is mountedcoaxially within the housing and a thin resistive film is deposited onthe outer surface thereof and series connected between the contacts ofthe connector to convert electrical energy being transmitted by thetransmission line into heat energy. The housing defines a closed annularinterior horn chamber surrounding the resistive film for minimizing thereflected energy from the dummy load.

In accordance with the invention, inner and outer cylindrical sleeveelements formed of low-friction insulating material such as TEFLON(polytetrafluoroeythylene) are provided to define inner and outerannular cylindrical liquid flow passages coaxial with the housing. Theinner annular flow passage is connected to a liquid inlet at one end andsurrounds the resistive film along the entire length thereof to absorbheat energy therefrom. The outer annular flow passage communicates withthe inner annular passage through radial ports at the end thereofopposite the inlet fitting and extends back to an outlet chamber with anoutlet fitting at the same end as the inlet fitting.

Normally a heat exchanger and pump are provided to complete the coolingsystem. The space within the di electric member is most advantageouslyprovided with a rod-like filler element adapted and designed to helpprovide optimum electrical characteristics for the device. An annularspace between the filler rod and the interior wall of the cylindricalceramic dielectric member is filled with liquid coolant by tapping fromthe inlet, the water being gradually replenished by means of one or moresmall exhaust ports. It will be noted that it is not necessary, however,to maintain circulation through the interior of the dielectric membersince the cooling function is accomplished by' liquid coolant flowingthrough the annular flow passages.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view along theaxis of a liquid cooled dummy load device embodying the invention andincluding certain associated equipment illustrated in diagrammatic form;

FIG. 2 is a sectional view taken on the line 2-2 of FIG. 1;

FIG. 3 is a fragmentary sectional view taken on the line 33 of FIG. land with parts broken away for the purpose of illustration; and

FIG. 4 is a fragmentary sectional view taken on the line 4-4 of FIG. 1.

DESCRIPTION OF THE PREFERRED I EMBODIMENT Referring more particularly tothe drawings and initially to FIG. 1 there is shown a system forterminating a coaxial RF transmission line. The system includes areflectionless type dummy load device with integral means fordissipating the heat generated across the load. A signal generator 11 isconnected by a coaxial transmission line including an inner conductor 12and an outer conductor 13, to the device with a suitable connector. Thesystem also includes a heat exchanger 15 and a pump 16., both seriallyconnected to the de vice 10 by an inlet pipe 17 and an outlet pipe 18.The pump 16 circulates liquid coolant (e.g., water) between the device10 and the heat exchanger 15 and provides the necessary flow velocity toachieve the necessary cooling. The signal generator 11 produces signalsat any frequency from DC up to the microwave region and most typicallyit constitutes a transmitter.

As will be apparent from FIG. 1 the device is constructed so that thefittings for cooling fluid connections are all at the right hand orupper end of the device whereas the electrical connections are made atthe left hand or lower end of the device which will be referred toherein as the power connection end. The power connection end includes aninner connector sleeve 21 adapted to receive the inner conductor 12 ofthe transmission line and an outer connector flange 22 which is adaptedto .receive the outer coaxial conductor 13 of the transmission line. Theouter connector flange 22 functions as a swivel follower and is brazedto an outer connector sleeve 23 coaxial with the inner connector sleeve21.

An annular swivel plate 24 is telescoped over the outer connector flange22 and has a plurality of circumferentially spaced holes that receivemounting bolts for securing the device 10 to appropriate structure. Theswivel follower flange 22 may be rotated relative to the mounting plate24 to position the device about its axis as desired.

Another annular flange 25 is brazed to the opposite end of the outerconnector sleeve 23, and is adapted to be bolted to another annularflange 26. The flange 26 is brazed to the main body of the device andspecifi cally to a main housing section 30 which constitutes one of thethree housing sections including a secondary housing section 31 and ahousing end section 32.

The assembly within the housing includes outer and inner flow tubes 33and 34 respectively formed of TEF- LON or other suitable electricalinsulating material with a low-friction characteristic; a dielectriccylinder 35 within the flow tube 33, and with a thin resistive filmcoated on the outer surface thereof, an insert rod 36 within the sleeve35 (TEFLON). a resistor socket 37 (TEFLON) anda:'resistor ground fitting38 (TEF- LON), all of which are coaxially mounted relative to oneanother. The main housing section 30 has an upper (right hand) end withexternal threads adapted to be engaged by internal threads formed at thelower (left hand) end of the secondary housing section 31. The upper(right hand) end of the secondary housing section3l is also providedwith internal threads adapted to be engaged by external threads formedin the housing end section 32.

The lower (left hand) end of the outer flow tube 33 is received in anannular recess 42 formed in the resistor socket 37, and is provided withan inwardly extending flange 41. The upper (right hand) end of the outerflow tube 33 seats snugly within the inner surface of the main housingsection 30 and the respective surfaces are sealed by an O-ring seal 48.An inwardly extending flange 43 at the upper (right hand) end of themain housing section 30 serves to retain the outer flow tube 33 in itsaxial position between the flange 43 and the resistor socket 37.

A fluid tight seal is maintained between the flange 41 on the outer flowtube 33 and the conductor 44 by means of an O-ring seal 47 which isreceived in an an nular groove in the flange 41.

A conductor rod 44 of a generally cylindrical form extends through theresistor socket 37 and the flange 41 on the outer flow tube 33 and has athreaded stud which is used to bolt the inner conductor sleeve 21thereto with a nut 46. This places the inner conductor sleeve 21 inelectrical contact with the conductor rod 44.

The inner flow tube 34 is positioned and radially spaced within theouter flow tube 33 with a spacer ring 49.'The inner flow tube 34 extendsaxially to the upper (right hand) end of the device and is sealed at theupper end against the secondary housing section 31 using an O-ring seal50 which is positioned in an annular groove located in the inner surfaceof the secondary housing section 31. The inner flow tube 34 is retainedin its axial position by the resistor ground fitting 38 which seatsagainst the upper (right hand) end of the inner flow tube 34 and thusretains its opposite end against the flange 41 of the outer flow tube33. The resistor ground fitting 38 has a radial flange 51 that seats ina matching annular recess 52 in the secondary housing section 31 and isretained therein by the lower (left hand) end of the housing end section32. The housing end section 32 is sealed against the adjacent portion ofthe secondary housing section 31 by still another 0- ring seal.

As will be apparent from FIG. 1 the resistor ground fittting 31 and thehousing end section 32 define a liquid cooland inlet chanber 53thatcommunicates with the inlet pipe 17 through an axial fitting 54 threadedinto a matching threaded receptacle in the end of the housing endsection 32. The resistor ground fitting 38 is provided with radial ports55 that communicate between the in'let chamber 53 and an inner annularflow chamber 56=defined between the dielectric cylinder'35 and the innerflow tube 34. Liquid coolant flowing through the-inner annular flowchamber 56 is in intimate contact with the resistive film on thedielectric sleeve 35 and thus absorbs heat generated by the dissi pationof electrical energy.

From the inner annular flow chamber 56 liquid coolant flows radiallyoutward to an outer annular flow chamber 57 through radial ports 58 inthe inner flow tube 34. The outer annular flow chamber 57 is coaxialwith the inner annular flow chamber 56 and is defined by the outersurface of the inner flow tube 34 and the inner surface of the outerflow tube 33.

Liquid coolant proceeds through the inner annular flow chamber 57 asindicated by the flow arrows in FIG. 1 and then through radial ports 59in the main housing section 30 to an annular liquid coolant outletchamber 60 defined between the outer surface of the main housing section30 and the inner surface of the secondary housing section 31. Liquidcoolant exits the water outlet chamber 60 to the water outlet pipe 18through an outlet fitting 61 extending through the wall of the secondaryhousing section 31.

The interior wall at the lower (left hand) end of the main housingsection 30 is formed with a tapered exponential surface of revolution tominimize reflection from the device and to reduce the VSWR. Theelectrical characteristics of the device as thus designed are inbalancing relation so that the Z,, of the device varies from that of thetransmission line down to zero.

As indicated above an insert rod 36 formed, for ex ample of TEFLON, ispositioned within the dielectric sleeve 35 in accordance with theteachings of U.S. Pat. No. 3,300,737 to assure that the desiredelectrical characteristics are achieved. The diameter of the rod 36 isslightly less than the inner diameter of the dielectric cylinder 35 sothat a thin annular space 63 is defined therebetween. In order toachieve the desired electrical characteristics for the device, the space63 is filled with liquid coolant from the inlet chamber 53. The coolantenters through an axial opening 65 (FIGS. 3 and 4) in the end of theresistor ground fitting 38 and then radially outward through a slot 62formed in the end of the insert rod 36 to the space 63.

While it is not necessary to circulate the coolant through the space 63,some movement is beneficial and accordingly a radial bleed port 66 isprovided in the resistor socket 37 to permit some escape of coolant andthus a small amount of continuous movement of the liquid coolantindicated by the flow arrows in FIG. 1.

With the construction thus described, the liquid coolant contacts andabsorbs heat energy from the resistive film on the ceramic element 35during the initial part of its circulation through the device and duringan interval when it is at its highest velocity. This affords a moreefficient and more uniform heat transfer. An ad ditional advantage isthat compared with prior art devices the device of the invention affordsa much lower resistance to fluid flow and thus a much lower pressuredrop. By forming the two annular passages 56 and 57 with sleevescomprising a low friction material, resistance to liquid flow isminimized and the device provides a minimum pressure drop for thecooling liquid being pumped therethrough. This factor is extremelyimportant in view of the high flow velocities that must be used in orderto afford the necessary heat tranfer.

While the invention has been shown and described with respect to aspecific embodiment thereof, this is intended for the purposeofillustration rather than limitation and other modifications andvariations of the specific machine herein shown and described will beapparent to those skilled in the art all within the in tended scope andspirit of the invention. Accordingly, the patent is not to be limited tothe specific embodi ment herein shown and described nor in any other waythat is inconsistent with the extend to which the progress in the arthas been advanced by the invention.

1 claim:

1. A liquid cooled dummy load device for an RF coaxial transmissionline, the device having a connector with first and second contact meansfor the inner and outer coaxial conductors respectively, and acylindrical housing;

an elongated tubular dielectric member coaxially mounted in saidhousing,

a thin resistive film deposited on the outer surface of said dielectricmember and series connected between said first and second contact means,said film being adapted to convert electrical energy being transmittedby said transmission line to heat energy,

means defining a liquid inlet, and a liquid outlet at one end of saidhousing,

a first cylindrical sleeve member formed of low friction insulatingmaterial surrounding and coaxial with said tubular dielectric member anddefining with said thin resistive film an inner annular cylindricalliquid flow passage communicating with said liquid inlet,

a second cylindrical sleeve member formed of low friction insulatingmaterial surrounding and coaxial with said first cylindrical sleevemember and defining with the outer surface of said first cylindricalsleeve member an outer annular liquid fiow passage communicating withsaid liquid outlet, said second cylindrical sleeve member defining withsaid housing an exponential horn chamber adapted to contain an airmedium for minimizing the re flected energy from said termination,

means defining radial ports connecting said inner liquid flow passage tosaid outer liquid flow passage at the ends thereof opposite said inletand outlet,

whereby liquid flowing in said inner flow passage ab sorbs heat energyfrom said resistive film,

means defining a liquid chamber within said tubular dielectric memberand communicating with said liquid inlet to provide a liquid dielectricmedium within said tubular dielectric member, and

means for pumping cooling liquid through said passages.

2. A device as defined in claim 1 wherein said first and second sleevemembers are formed of polytetrafluoro ethylene.

3. A device as defined in claim 1 wherein said tubular dielectric memberdefines a radial bleed port at the end thereof opposite said inlet andoutlet whereby liquid coolant in said liquid chamber bleeds out intosaid inner flow passage and is continously replenished.

1. A liquid cooled dummy load device for an RF coaxial transmissionline, the device having a connector with first and second contact meansfor the inner and outer coaxial conductors respectively, and acylindrical housing; an elongated tubular dielectric member coaxiallymounted in said housing, a thin resistive film deposited on the outersurface of said dielectric member and series connected between saidfirst and second contact means, said film being adapted to convertelectrical energy being transmitted by said transmission line to heatenergy, means defining a liquid inlet, and a liquid outlet at one end ofsaid housing, a first cylindrical sleeve member formed of low frictioninsulating material surrounding and coaxial with said tubular dielectricmember and defining with said thin resistive film an inner annularcylindrical liquid flow passage communicating with said liquid inlet, asecond cylindrical sleeve member formed of low friction insulatingmaterial surrounding and coaxial with said first cylindrical sleevemember and defining with the outer surface of said first cylindricalsleeve member an outer annular liquid flow passage communicating withsaid liquid outlet, said second cylindrical sleeve member defining withsaid housing an exponentIal horn chamber adapted to contain an airmedium for minimizing the reflected energy from said termination, meansdefining radial ports connecting said inner liquid flow passage to saidouter liquid flow passage at the ends thereof opposite said inlet andoutlet, whereby liquid flowing in said inner flow passage absorbs heatenergy from said resistive film, means defining a liquid chamber withinsaid tubular dielectric member and communicating with said liquid inletto provide a liquid dielectric medium within said tubular dielectricmember, and means for pumping cooling liquid through said passages.
 2. Adevice as defined in claim 1 wherein said first and second sleevemembers are formed of polytetrafluoro ethylene.
 3. A device as definedin claim 1 wherein said tubular dielectric member defines a radial bleedport at the end thereof opposite said inlet and outlet whereby liquidcoolant in said liquid chamber bleeds out into said inner flow passageand is continously replenished.